Full-length HGF isoforms are each synthesized as a single polypeptide chain, pre-pro-HGF, containing an amino-terminal signal peptide sequence for insertion into the rough endoplasmic reticulum (RER) and ultimately, secretion. Maturation of pre-pro-HGF is presumed to follow a conventional subcellular pathway for secreted proteins, i.e. from RER to the Golgi apparatus to secretory vesicles that ultimately fuse with the plasma membrane allowing protein release into the extracellular environment. There is evidence for both N-linked (Hara et al., 1993) and O-linked glycosylation (Shimizu et al., 1992) of HGF during maturation, and presumably removal amino-terminal 31 amino acid signal peptide occurs prior to secretion (Miyazawa et al., 1991). The secreted single chain HGF precursor (pro-HGF) is biologically inactive and later converted in the active two-chain disulfide-linked heterodimer by proteolytic cleavage (as described above) in the extracellular space, in plasma, or on target cell surfaces.

Embryonic development. HGF and its receptor, Met, are expressed during gastrulation and throughout later phases of vertebrate embryogenesis (Stern et al., 1990; Sonnenberg et al., 1993; Andermarcher et al., 1996). Overlapping expression of both genes persists into the earliest phases of organogenesis in the heart, condensing somites and neural crest cells (Andermarcher et al., 1996), but thereafter HGF is expressed in mesenchymal tissues and Met in the surrounding ectoderm in differentiated somites as well as lungs, liver, placenta, muscle, gut, heart and nervous system (Sonnenberg et al., 1993; Woolf et al., 1995; Andermarcher et al., 1996; Thewke and Seeds, 1996; Birchmeier and Gherardi, 1998; Ishikawa et al., 2001). Studies using tissue explants and cultured cells confirm the suspected role of HGF in epithelial branching morphogenesis, e.g. in the developing lung (Santos et al., 1994; Woolf et al., 1995; Ohmichi et al., 1998). The expression of HGF and Met genes in ventral motor neurons of the embryonic spinal cord is also consistent with a role in tissue patterning through the regulation of migratory and morphogenic processes, such as axon guidance (Sonnenberg et al., 1993; Ebens et al., 1996; Wong et al., 1997). Functional studies indicate that HGF guides axons of spinal motor neurons to their distant muscle targets in the limbs (Ebens et al., 1996; Wong et al., 1997; Yamamoto et al., 1997) and acts as an essential survival factor for a subpopulation of limb-innervating motoneurons (Wong et al., 1997; Yamamoto et al., 1997). Both HGF and Met are also expressed in the brain and retina during development (E12-13) and in the adult, where signaling supports neuron survival and maturation (Jung et al., 1994; Honda et al., 1995; Yamagata et al., 1995; Hamanoue et al., 1996; Achim et al., 1997; Sun et al., 1999; Thewke and Seeds, 1999). Loss of HGF or Met function in mice with homozygous gene deletion is embryonic lethal between days E12.5 and E15.5 (Schmidt et al., 1995; Uehara et al., 1995; Bladt et al., 1995). Defects in the proliferation and survival of cells in the liver and placenta result in arrested organogenesis of these and other tissues, underscoring the importance of HGF stimulated mitogenicity and survival in target cells. These models also highlight the importance of HGF as a potent and critical regulator of cell migration. Skeletal muscle progenitor cells that form limb, tongue, and diaphragm musculature normally delaminate from the epithelial dermomyotome of the somites by an epithelial-to-mesenchymal transition and migrate to their final destination where they complete differentiation. Homozygous deletion of Met results in defective delamination and migration of muscle progenitors from the dermomyotome and failure to form the skeletal muscles of the limb and diaphragm (Bladt et al., 1995; Maina et al., 1996; Dietrich et al., 1999; Rosário and Birchmeier, 2003; Christ and Brand-Saberi, 2002). Conversely, HGF overexpression in transgenic mouse embryos induces the inappropriate formation of skeletal muscle in the central nervous system (CNS) through dysregulated migration of Met containing myogenic precursor cells to the neural tube (Takayama et al., 1996). Mice bearing conditional deletions of HGF or Met also have been used to demonstrate relevance of pathway activation at later developmental stages and in adulthood. Met and epidermal growth factor receptor jointly regulate final nephron number and collecting duct morphology (Ishibe et al., 2009). Mice with a targeted mutation of the gene encoding urokinase plasminogen activator, considered an important HGF activator, have decreased HGF levels and a substantial reduction in neocortical GABAergic interneurons at embryonic and perinatal ages, leading to changes in circuit organization and behavior (Powell et al., 2001; Powell et al., 2003a). Mice with targeted mutation of two critical carboxyl terminal tyrosine residues in Met were found to be phenotypically similar to Met null animals. In contrast, targeting one of those sites and thereby disrupting the consensus for Grb2 binding allowed development to proceed to term, but caused a striking reduction in limb muscle mass and a generalized deficit of secondary fibers, indicating the importance of HGF signaling in late myogenesis (Maina et al., 1996).

Maturity and adult homeostasis. In the developed brain, HGF is expressed in neurons, primarily in the hippocampus, cortex, and the granule cell layer of the cerebellum, as well as in ependymal cells, the chorioid plexus, and the pineal body (Streit et al., 1995). Met is expressed in neurons, preferentially in the CA-1 area of the hippocampus, the cortex, and the septum, as well as in the pons (Jung et al., 1994; Streit et al., 1995; Honda et al., 1995; Yamagata et al., 1995; Thewke and Seeds, 1999). HGF is though to provide a neurotrophic function in the CNS, supporting the survival and reconstruction of specific neurons in response to cerebral injury (Honda et al., 1995). HGF attracts and promotes the growth of cranial motor axons (Caton et al., 2000), induces c-Fos expression and activates the Ras pathway in brain neurons (Streit et al., 1997), stimulates Schwann cell growth (Krasnoselsky et al., 1994) and promotes axon outgrowth of embryonal carcinoma cells (Yang and Park, 1993). HGF stimulates neurite outgrowth in sensory and sympathogenic neurons, as well as enhanced survival and differentiation from progenitors (Maina et al., 1997; Maina et al., 1998). HGF and Met are expressed in the cerebellum, where development is primarily postnatal and requires extensive cell proliferation and migration. Met is localized in granule cell precursors and cultures of these cells proliferate in response to HGF (Ieraci et al., 2002). HGF also promotes oligodendrocyte progenitor cell proliferation and delays their differentiation into myelinating oligodendrocytes during early postnatal development; subsequent down-regulation of HGF mRNA in the striatum observed between postnatal days 7 to 14 presumably permits differentiation and myelination to proceed (Ohya et al., 2007). Schwann cells, responsible for nerve myelination in the peripheral nervous system, also express Met mRNA (Krasnoselsky et al., 1994). Although Schwann cells are normally quiescent in adulthood, nerve injury and certain diseases such as type 1 neurofibromatosis trigger proliferation through several mitogenic pathways, including that of HGF (Krasnoselsky et al., 1994). The mammary gland undergoes cyclic morphogenic differentiation during the menstrual cycle, pregnancy and lactation. HGF and Met are expressed and HGF is regulated temporally during mouse mammary development and differentiation (Niranjan et al., 1995; Yang et al., 1995). HGF secreted by fibroblasts acts on mammary myoepithelial and luminal epithelial cells expressing Met, promoting tubulogenesis in underlying myoepithelial cells, branching of the epithelial ductal tree and motogenesis in both cell types (Niranjan et al., 1995; Yang et al., 1995; Yant et al., 1998; Niemann et al., 1998). HGF production in the adult vascular system is positively regulated by prostaglandins and HGF itself, and negatively regulated by angiotensin II, TGF-beta, glucose and hypoxia (reviewed in Morishita et al., 2002). HGF is induced in cardiac and skeletal muscle in animal models of ischemic injury (Aoki et al., 2000) and serum HGF levels are increased with hypertension, peripheral artery disease and myocardial infarction, consistent with homeostatic and repair functions (reviewed in Morishita et al., 2002).

Human HGF is highly conserved among mammals but (99.9% amino acid identity between human and chimp, to 91% between human and rat), however, homologs rapidly diverge in birds (75% between human and chicken) and bony fish (50% between human and zebra fish). Structural homology beyond teleosts is partial. More generally, HGF resembles members of the plasminogen family (~38% amino acid identity), in that the mature 2-chain protein contains multiple kringle domains in the amino terminal alpha (or heavy) chain and a serine protease like domain in the carboxyl terminal beta (or light) chain. Unlike the canonical plasminogen family members, HGF is devoid of proteolytic activity (reviewed in Matsumoto and Nakamura, 1996). Of plasminogen family members, HGF is most closely related to macrophage stimulating protein (MSP; 44% amino acid identity; also known as MST1 or HGF-like protein).

Overexpression of both human HGF and MET is found in most papillary thyroid carcinomas, but not other thyroid tumor types. At least one study reported that the majority of these cases appear to possess autocrine HGF/Met signaling (Trovato et al., 1998) although this is controversial (Oyama et al., 1998). Increased MET and HGF expression is associated with a high risk for metastasis and recurrence in children and young adults with PTC (Ramirez et al., 2000). Cell lines established from thyroid carcinomas respond to HGF with increased motility and invasiveness, increased chemokine and VEGF production, and the recruitment of dendritic cells and new blood vessels (de Luca et al., 1999; Scarpino et al., 1999; Scarpino et al., 2000; Scarpino et al., 2003).

Analysis of breast tumor HGF levels in a large cohort revealed that patients with high values had a significantly shorter relapse-free survival and overall survival when compared to those with low values; in fact, HGF levels were a better independent predictor of relapse-free and overall survival than lymph node involvement (Yamashita et al., 1994; Nagy et al., 1996). Serum HGF levels were also significantly higher than those of healthy controls in about one-third of breast cancer patients, a finding significantly associated with node status, tumor size and histological evidence of venous invasion (Taniguchi et al., 1995; Toi et al., 1998; Sheen-Chen et al., 2005). Removal of the primary tumor decreased the serum HGF levels, suggesting that the elevation was tumor-related (Taniguchi et al., 1995). Almost all patients with recurrent breast cancer also had increased serum HGF level, and patients with liver metastases had higher levels compared to those with other sites of metastases (Taniguchi et al., 1995; Maemura et al., 1998; Eichbaum et al., 2007). Somatic mutations and functional polymorphisms in the HGF gene promoter cause increased HGF production in breast cancer; 51% of African Americans and 15% of individuals of mixed European descent with breast cancer harbor a promoter truncation variant in their breast tumors that which is associated with increased cancer incidence and a substantially younger age of disease onset than those with a wild-type genotype (Ma et al., 2009).

Inherited missense mutations in the human HGF receptor gene were first found in individuals with hereditary papillary renal carcinoma (HPRC) type 1; similar somatic mutations were also found in a small subset of (PRC) tumor samples (reviewed in Dharmawardana et al., 2004). Trisomy of human chromosome 7, which contains both Met and HGF genes, occurs in 95% of sporadic papillary renal carcinoma and nearly all HPRC cases, where there is always non-random duplication of the mutant allele. Although the role of HGF in the oncogenicity of HPRC and PRC-associated Met mutations is not yet defined, ligand binding clearly promotes cell transformation (Michieli et al., 1999).

The binding properties of minimal oligosaccharides reveal a common heparan sulfate/dermatan sulfate-binding site in hepatocyte growth factor/scatter factor that can accommodate a wide variety of sulfation patterns.

Analysis of deleted variant of hepatocyte growth factor by alanine scanning mutagenesis: identification of residues essential for its biological function and generation of mutants with enhanced mitogenic activity on rat hepatocytes.

Results of a double-blind, placebo-controlled study to assess the safety of intramuscular injection of hepatocyte growth factor plasmid to improve limb perfusion in patients with critical limb ischemia.

Over-expression of hepatocyte growth factor/scatter factor (HGF/SF) and the HGF/SF receptor (cMET) are associated with a high risk of metastasis and recurrence for children and young adults with papillary thyroid carcinoma.

The expression of mRNAs for hepatocyte growth factor/scatter factor, its receptor c-met, and one of its activators tissue-type plasminogen activator show a systematic relationship in the developing and adult cerebral cortex and hippocampus.